1. Field of the Invention
Some embodiments of the invention are systems and methods for calibrating a display using a camera device (e.g., a handheld camera device) to measure light emitted by the display in a manner emulating measurements by a reference camera having known sensitivity function but without preknowledge of the sensitivity function of the camera device's camera. In typical embodiments, preprocessor control parameters determined using a handheld or camera device are asserted as feedback to a video preprocessor to recalibrate a display.
2. Background of the Invention
Throughout this disclosure including in the claims, the expression performing an operation “on” signals or data (e.g., filtering or scaling the signals or data) is used in a broad sense to denote performing the operation directly on the signals or data, or on processed versions of the signals or data (e.g., on versions of the signals that have undergone preliminary filtering prior to performance of the operation thereon).
Throughout this disclosure including in the claims, the expression “system” is used in a broad sense to denote a device, system, or subsystem. For example, a subsystem that implements a filter may be referred to as a filter system, and a system including such a subsystem (e.g., a system that generates X output signals in response to multiple inputs, in which the subsystem generates M of the inputs and the other X-M inputs are received from an external source) may also be referred to as a filter system.
Throughout this disclosure including in the claims, the noun “display” and the expression “display device” are used as synonyms to denote any device or system operable to display an image or to display video in response to an input signal. Examples of displays are computer monitors, television sets, and home entertainment system monitors or projectors.
Throughout this disclosure including in the claims, the terms “calibration” and “recalibration” of a display denote adjusting at least one parameter or characteristic of the display, e.g., a color, brightness, contrast, and/or dynamic range characteristic of the display. For example, recalibration of a display device can be implemented by performing preprocessing on input image data (to be displayed by the display device) to cause the light emitted by the display device in response to the preprocessed image data (typically after further processing is performed thereon) to have one or more predetermined color, brightness, contrast, and/or dynamic range characteristics.
Throughout this disclosure including in the claims, the term “processor” is used in a broad sense to denote a system or device programmable or otherwise configurable (e.g., with software or firmware) to perform operations on data (e.g., video or other image data). Examples of processors include a field-programmable gate array (or other configurable integrated circuit or chip set), a digital signal processor programmed and/or otherwise configured to perform pipelined processing on video or other image data, a programmable general purpose processor or computer, and a programmable microprocessor chip or chip set.
Throughout this disclosure including in the claims, measured “light intensity” is used in a broad sense, and can denote measured luminance or another measured indication of light intensity appropriate in the context in which the expression is used.
Throughout this disclosure including in the claims, the term “camera” is used in a broad sense to denote a light sensor (e.g., a colorimeter or other sensor whose output can be analyzed to determine a color or frequency spectrum of sensed light), or a camera including an image sensor array (e.g., a CCD camera), or a camera of any other type. Typical embodiments of the invention employ a handheld camera device which includes a camera operable to sense an image displayed by a monitor or other display and to output data indicative of the sensed image (or one or more pixels thereof).
Throughout this disclosure including in the claims, the expression “camera device” denotes a device which includes (e.g., is) a camera and a processor coupled to receive the camera's output, and which is operable to measure at least one characteristic of light emitted by a display device (e.g., while the display device displays at least one test image) in a manner emulating measurement of the same light by a reference camera having known sensitivity function but without preknowledge of the sensitivity function of the camera device's camera. For example, a mobile phone which includes a camera and a processor coupled to receive the camera's output may be a camera device as defined in this paragraph. Typical embodiments of the invention include or employ a camera device which is a handheld device (“HHD”) or other portable device. Other embodiments of the invention include or employ a camera device which is not readily portable. In typical embodiments of the invention, a camera device (e.g., implemented as an HHD) is operable to download data indicative of a prior characterization or calibration of a display (e.g., data indicative of a sensitivity function of a reference camera employed to perform the prior characterization or calibration) and to measure at least one characteristic of light emitted by the display using the camera device's camera and the downloaded data in connection with a recalibration of the display. In a display characterizing operation (preliminary to color calibration of a display using a camera device in some embodiments of the invention), a reference camera having a known sensitivity function is used to measure the display's output as a function of wavelength in response to test colors and a white point. A set of reference values (e.g., values of a transfer function that matches the display's response for each test color and white point to the reference camera's response, and values of the reference camera's sensitivity function) are stored and later provided to the camera device, so that the camera device's output in response to light emitted by the display (e.g., during display of at least one test image) can be used with the reference values to emulate measurement of the same light by the reference camera.
It is conventional for a user to manually adjust controls of a display device to adjust or calibrate the device while the device displays test patterns (e.g., in response to test pattern data read from a DVD or other disk). While a display device displays test patterns, it is also conventional to use a colorimeter or camera to generate data that characterize the display device and/or data indicative of recommended settings for adjusting or calibrating the display device (e.g., to match target settings). With knowledge of such data, a user can manually adjust (or enter commands which cause adjustment of) controls of the display device to obtain a visually pleasing and/or acceptable displayed image appearance or to match target settings. It is also conventional to use such data to generate control values, and to assert the control values to a graphics card of the display device to calibrate the display device. For example, it is known to use a computer programmed with appropriate software to generate control values which determine look-up tables (LUTs) in response to such data and to assert the control values to the graphics card (e.g., to match target settings previously provided to the computer).
In professional reference environments (e.g., studios and post production facilities), such conventional techniques can be used to calibrate a display for use as a reference to grade content and adjust color, brightness, contrast, and/or tint parameters of content. An off-calibrated display can lead to dire consequences in the production environment and repair and/or recalibration can be very expensive. In such environments, there is a need for a closed-loop, carefully characterized measurement system that can automatically correct for variations in display calibration.
There is also a need for a closed-loop, carefully characterized measurement system that can automatically correct for variations in calibration of displays in a variety of environments (e.g., home entertainment system displays, and displays of home or business computer systems) without the need for the user to employ a highly calibrated imaging colorimeter (such colorimeters are typically expensive and difficult to set up) or other expensive, calibrated light or image sensor(s). Displays often need to be recalibrated in the field (e.g., in consumers' homes) with minimal field support, and often need to adapt to different external lighting environments. It had not been known before the present invention how to implement such a system with a camera device whose camera has a sensitivity function that is unknown “a priori” (e.g., an inexpensive handheld camera device including an inexpensive, uncalibrated camera) but which is operable to measure light emitted by a display in a manner emulating measurements by a reference camera having a known sensitivity function (e.g., an expensive, highly calibrated imaging colorimeter).
There is also a need for a closed-loop, carefully characterized measurement and calibration system that can automatically and dynamically correct for variations in calibration of a display, where the display is not configured to be calibrated (e.g., recalibrated) automatically in response to control signals generated automatically (without human user intervention) in response to camera measurements of light emitted by the display. For example, such a display may be configured to be recalibrated only in response to a human user's manual adjustment of color, brightness, contrast, and/or tint controls, or it may be the display device of a computer system that can be adjusted or recalibrated only in response to commands entered by human user by manually actuating an input device of the system (e.g., by entering mouse clicks while viewing a displayed user interface). Displays of this type often need to be recalibrated in the field with minimal field support, and should dynamically adapt to different external lighting environments. However, it had not been known before the present invention how to implement a closed-loop, carefully characterized measurement system to automatically correct for variations in calibration of a display of this type (including variations resulting from changes in external lighting environment).